The dihydric phenols have achieved significant success in their commercial applications. Dihydric phenols are useful in the commercial manufacture of various polymers including the polyarylates, polyamides, epoxies, polyetherimides, polysulfones and the polycarbonates. Significant attention has been directed to the commercial preparations of the dihydric phenols. For many years it has been well known that the acid catalyzed raction of phenol with specific aldehyde or ketone could prepare the 4,4'-dihydric phenol with specific groups derived from the aldehyde or the ketone connecting the two phenolic rings. In particular when phenol is reacted with acetone, the dihydric phenol 4,4'(hydroxyphenyl)propane-2, hereafter known as bisphenol-A is formed. This has particular utility in polycarbonates, polyarylates and copolyestercarbonates as well as epoxies. In order to make certain polymers, in particular the polycarbonates, the bisphenol-A must be particularly pure, for example, as measured by color. Additionally, the process should be particularly efficient since the dihydric phenol costs contribute substantially to the cost of the final polymer. Therefore much attention has been directed to the recovery of bisphenol-A after preparation. Not only is recovery from the major stream containing primarily bisphenol-A important, but because of the economics involved, various side streams or "purge streams" also contain significant quantities of bisphenol-A and should also be processed in manners which maximize bisphenol-A recovery.
Various catalytic systems for acid catalysis of the reaction between phenol and acetone have been investigated and used commercially. The hydrochloric acid catalyzed process is used in a significant number of commercial facilities. However the corrosion caused by the hydrochloric acid on reactors and pre and post reaction equipment leaves much to be desired as far as economics is concerned. Recently, substantial attention has been placed on using ion exchange resin catalyst systems since they do not have significant acid corrosion problems.
Various tactics have been utilized to maximize the quality and quantity of bisphenol-A recovered from the acidic ion exchange resin catalyzed reaction of phenol and acetone. U.S. Pat. No. 4,847,433 utilizes a carbonate metal oxidation number plus two salts of carbonates, to stabilize the bisphenol-A so that significant quantities of quality bisphenol-A can be recovered from various streams. It was thought that the specific acidic material that was being counteracted by the addition of the carbonate salts were minute quantities of strong acid oligomers which were being leached from the resin catalyst during the processing. It was noted that such carbonate salts should not be recycled to the catalyst system since they would very well bring about eventual neutralization of the catalyst system.
U.S. Pat. No. 4,894,486 specifically states that the presence of metal ions is also thought to have an adverse effect on the color of bisphenols probably by promoting degradation. The British Patent 890432 is then cited to show that various other additives have been employed to inhibit the formation of degradation products of the bisphenols. Thus, alkaline earth phosphates, stannic oxide and oxylate, a mixture of tin powder and tin dioxide, terephthalic and isophthalic acids, oxalic, sebacic and adipic acids and boron or antimony trioxides and their mixtures are taught as useful additives for providing thermal stabilities to bisphenols. Additionally in British Patent 890432 is mentioned the concept of utilizing a neutral or amphoteric compound or compound of weakly acidic character and possibly also possessing the property of forming complexes with metallic ions and ability to react with alkaline reacting impurities in the bisphenols is also mentioned. A further British Patent 1022583 teaches that improved color stability of bisphenols is provided by the incorporation of oxalic, citric or tartaric acids or their alkali metal or ammonium salts during a bisphenol manufacturing process. They may be added with the reactants or after the reaction is complete but before the bisphenol is separated from the reaction mixture. The British patents disclose acidic conditions for preparing bisphenol-A but no mention of acid ion exchange resin catalysis is mentioned.
Recently, U.S. Pat. No. 4,894,486 disclosed the use of the hydroxy acids lactic, malic and glyceric and their ammonium or alkali metal salts as stabilizers for bisphenols. No particular preparation of the bisphenol-A was employed and the only examples utilized the acid per se and measured the APHA color before and after heat treatment.
Weakly basic anion exchange columns have also been utilized to contact bisphenol containing fluids. In U.S. Pat. No. 4,191,843, a weakly basic anion exchange resin is used to contact reactor effluent obtained from an acid ion exchange resin catalyst. Instead of the weakly basic anion exchange resin, strongly acidic ion exchange resin in its salt form can also be used. U.S. Pat. No. 4,766,254, utilizes a weakly basic anion exchange resin to contact the mother liquor of bisphenol-A phenol adduct.
As can be seen from this virtual potpourri of prior art there is very little distinction given to the types of impurities which are being addressed in the manufacturing process of bisphenols, particularly bisphenol-A. The fact that any of acids, salts of acids, or bases can be used indicates that both alkaline and acidic impurities are being removed. Therefore there is no real directing nature to the prior art.
It is now been found that when utilizing an acidic ion exchange resin to catalyze a reaction between a phenol and a ketone to produce a bisphenol, particularly phenol and acetone to produce bisphenol-A, it is very advantageous to contact the desired bisphenol produced prior to significantly elevated temperatures with certain salts of particular weak organic acids. Examples of such temperatures include distillation of phenol or bisphenol as well as the separation of bisphenol from bisphenol phenol adduct. In this manner significant stabilization of the bisphenol, particularly bisphenol-A, is achieved when the bisphenol is subjected to a heat treatment, for example distillation of phenol or bisphenol or separation from its adduct of bisphenol with phenol. Degradation is significantly inhibited as shown by the substantial quantity of bisphenol which is capable of recovery and its enhanced color when the salts of the acid of this invention are in contact therewith.